diff --git a/src/mistralai/utils/serializers.py b/src/mistralai/utils/serializers.py
index 378a14c0..e74ee9c1 100644
--- a/src/mistralai/utils/serializers.py
+++ b/src/mistralai/utils/serializers.py
@@ -178,7 +178,7 @@ def is_nullable(field):
     if origin is Nullable or origin is OptionalNullable:
         return True
 
-    if not origin is Union or type(None) not in get_args(field):
+    if origin is not Union or type(None) not in get_args(field):
         return False
 
     for arg in get_args(field):
@@ -202,7 +202,11 @@ def stream_to_text(stream: httpx.Response) -> str:
 
 
 async def stream_to_text_async(stream: httpx.Response) -> str:
-    return "".join([chunk async for chunk in stream.aiter_text()])
+    # Use buffered accumulation for improved memory efficiency on large responses
+    buffer = []
+    async for chunk in stream.aiter_text():
+        buffer.append(chunk)
+    return "".join(buffer)
 
 
 def stream_to_bytes(stream: httpx.Response) -> bytes:
diff --git a/src/mistralai/utils/values.py b/src/mistralai/utils/values.py
index dae01a44..ac217f47 100644
--- a/src/mistralai/utils/values.py
+++ b/src/mistralai/utils/values.py
@@ -52,9 +52,11 @@ def match_status_codes(status_codes: List[str], status_code: int) -> bool:
 
 T = TypeVar("T")
 
+
 def cast_partial(typ):
     return partial(cast, typ)
 
+
 def get_global_from_env(
     value: Optional[T], env_key: str, type_cast: Callable[[str], T]
 ) -> Optional[T]: